Method for permanently bonding wafers

ABSTRACT

This invention relates to a method for bonding of a first contact surface of a first substrate to a second contact surface of a second substrate, the second substrate having at least one reaction layer, with the following steps, especially the following sequence:
         forming a reservoir in a reservoir formation layer on the first contact surface,   at least partial filling of the reservoir with a first educt or a first group of educts,   the first contact surface making contact with the second contact surface for formation of a prebond connection,   thinning of the second substrate and   forming a permanent bond between the first and second contact surface, at least partially strengthened by the reaction of the first educt with the second educt contained in the reaction layer of the second substrate.

FIELD OF THE INVENTION

This invention relates to a method for bonding of a first contact surface of a first substrate to a second contact surface of a second substrate.

BACKGROUND OF THE INVENTION

The objective in permanent or irreversible bonding of substrates is to produce an interconnection which is as strong and especially as irreversible as possible, i.e., a high bond force, between the two contact surfaces of the substrates. There are various approaches and production methods for this purpose in the prior art.

The known production methods and the approaches which have been followed to date often lead to results which cannot be reproduced or can be poorly reproduced and which can hardly be applied especially to altered conditions. In particular, production methods which are used at present often use high temperatures, especially >400° C., in order to ensure reproducible results.

Technical problems such as high energy consumption and a possible destruction of structures which are present on the substrates result from the high temperatures to some extent far above 300° C. which have been necessary to date for a high bond force.

Other demands consist in the following:

-   -   front-end-of-line compatibility.

This is defined as the compatibility of the process during the production of the electrically active components. The bonding process must therefore be designed such that active components such as transistors, which are already present on the structure wafers, are neither adversely affected nor damaged during the processing. Compatibility criteria include mainly the purity of certain chemical elements (mainly in CMOS structures) and mechanical loadability, mainly by thermal stresses.

-   -   low contamination.     -   no application of force.     -   temperature as low as possible, especially for materials with         different coefficients of thermal expansion.

The reduction of the bond force leads to more careful treatment of the structure wafer and thus to a reduction of the failure probability by direct mechanical loading.

The object of this invention is therefore to devise a method for careful production of a permanent bond having a bond force which is as high as possible at a temperature which is at the same time as low as possible.

SUMMARY OF THE INVENTION

This object is achieved with the features of claim 1. Advantageous developments of the invention are given in the dependent claims. All combinations of at least two of the features given in the specification, the claims and/or the figures also fall within the framework of the invention. At the given value ranges, values within the indicated limits will also be considered to be disclosed as boundary values and will be claimed in any combination.

The basic idea of this invention is to create a reservoir for holding a first educt at least on one of the substrates, the first educt after making contact or producing a temporary bond between the substrates reacts with a second educt which is present in the other substrate and thus forms an irreversible or permanent bond between the substrates. Before or after forming the reservoir in a reservoir formation layer on the first contact surface, generally cleaning of the substrate or substrates, especially by a flushing step, occurs. This cleaning should generally ensure that there are no particles on the surfaces which would result in unbonded sites. The reservoir, and the educt contained in the reservoir, create a technical possibility of inducing a reaction (first educt or first group with the second educt or the second group) which increases the bonding speed and strengthens the permanent bond directly on the contact surfaces after producing the temporary or reversible bond in a dedicated manner, especially by deforming at least one of the contact surfaces by the reaction, preferably the contact surface opposite the reservoir. On the opposing second contact surface as claimed in the invention there is a growth layer in which the deformation as claimed in the invention takes place and the first educt (or the first group) reacts with the second educt (or the second group) present in the reaction layer of the second substrate. To accelerate the reaction between the first educt (or the first group) and the second educt (or the second group) it is provided as claimed in the invention that the growth layer located between the reaction layer of the second substrate and the reservoir be thinned before the substrates make contact since in this way the distance between the reaction partners is reduced and at the same time the deformation/formation of the growth layer is promoted. The growth layer is removed at least partially, especially mostly, preferably completely, by the thinning. The growth layer grows again in the reaction of the first educt with the second educt even if it has been completely removed.

According to the invention there can be means for inhibiting the growth of the growth layer before the contact surfaces make contact, especially by passivation of the reaction layer of the second substrate, preferably by exposure to N₂, forming gas or an inert atmosphere or under a vacuum or by amorphization. In this connection treatment with plasma which contains forming gas, especially consists largely of forming gas, has proven especially suitable. Here forming gas is defined as gases which contain at least 2%, better 4%, ideally 10% or 15% hydrogen. The remaining portion of the mixture consists of an inert gas such as for example nitrogen or argon.

Alternatively or in addition to this measure, it is advantageous as claimed in the invention to minimize the time between the thinning and the contact-making, especially <2 hours, preferably <30 minutes, even more preferably <15 minutes, ideally <5 minutes. Thus the oxide growth which takes place after thinning can be minimized.

The diffusion rate of the educts through the growth layer is increased by the growth layer which has been thinned and thus is very thin at least at the beginning of the formation of the permanent bond or at the start of the reaction. This leads to a shorter transport time of the educts at the same temperature.

For the prebonding step, for producing a temporary or reversible bond between the substrates there are various possibilities with the objective of producing a weak interaction between the contact surfaces of the substrates. The prebond strengths are below the permanent bond strengths, at least by a factor of 2 to 3, especially by a factor of 5, preferably by a factor of 15, still more preferably by a factor of 25. As guideline values, the prebond strengths of pure, nonactivated, hydrophilized silicon with roughly 100 mJ/m² and of pure, plasma-activated hydrophilized silicon with roughly 200-300 mJ/m² are mentioned. The prebonds between the molecule-wetted substrates arise mainly due to the van-der-Waals interactions between the molecules of the different wafer sides. Accordingly, mainly molecules with permanent dipole moments are suitable for enabling prebonds between wafers. The following chemical compounds are mentioned as interconnect agents by way of example, but not limited thereto

-   -   water     -   thiols     -   AP3000     -   silanes and/or     -   silanols.

Substrates as claimed in the invention are those whose material is able to react as an educt with another supplied educt to form a product with a higher molar volume, as a result of which the formation of a growth layer on the substrate is caused. The following combinations are especially advantageous, to the left of the arrow the educt being named and to the right of the arrow, the product/products (without the supplied educt or byproducts which react with the educt being named in particular):

-   -   Si→SiO₂, Si₃N₄, SiN_(x)O_(y)     -   Ge→GeO₂, Ge₃N₄     -   a-Sn→SnO₂     -   B→B₂O₃, BN     -   Se→SeO₂     -   Te→TeO₂, TeO₃     -   Mg→MgO, Mg₃N₂     -   Al→Al₂O₃, AlN     -   Ti→TiO₂, TiN     -   V→V₂O₅     -   Mn→MnO, MnO₂, Mn₂O₃, Mn₂O₇, Mn₃O₄     -   Fe→FeO, Fe₂O₃, Fe₃O₄     -   Co→CoO, Co₃O₄,     -   Ni→NiO, Ni₂O₃     -   Cu→CuO, Cu₂O, Cu₃N     -   Zn→ZnO     -   Cr→CrN, Cr₂₃C₆, Cr₃C, Cr₇C₃, Cr₃C₂     -   Mo→Mo₃C₂     -   Ti→TiC     -   Nb→Nb₄C₃     -   Ta→Ta₄C₃     -   Zr→ZrC     -   Hf→HfC     -   V→V₄C₃, VC     -   W→W₂C, WC     -   Fe→Fe₃C, Fe₇C₃, Fe₂C

The following mixed forms of semiconductors are also conceivable as substrates:

-   -   III-V: GaP, GaAs, InP, InSb, InAs, GaSb, GaN, AlN, InN,         Al_(x)Ga_(I-x)As, In_(x)Ga_(I-x)N     -   IV-IV: SiC, SiGe,     -   III-IV: InAlP,     -   nonlinear optics: LiNbO₃, LiTaO₃, KDP (KH₂PO₄)     -   solar cells: CdS, CdSe, CdTe, CuInSe₂, CuInGaSe₂, CuInS₂,         CuInGaS₂     -   conductive oxides: In_(2-x)SnxO_(3-y)

According to the invention, on at least one of the wafers there is the reservoir (or reservoirs), directly on the respective contact surface, in which a certain amount of at least one of the supplied educts for the volume expansion reaction can be stored. Educts can therefore be for example O₂, O₃, H₂O, N₂, NH₃, H₂O₂, etc. Due to the expansion, especially dictated by oxide growth, based on the tendency of the reaction partners to reduce system energy, possible gaps, pores, and cavities between the contact surfaces are minimized and the bond force is increased accordingly by narrowing the distances between the substrates in these regions. In the best possible case the existing gaps, pores and cavities are completely closed so that the entire bonding area increases and thus the bond force rises accordingly.

The contact surfaces conventionally show a roughness with a quadratic roughness (R_(q)) of 0.2 nm. This corresponds to peak-to-peak values of the surfaces in the range of 1 nm. These empirical values were determined with Atomic Force Microscopy (AFM).

The reaction as claimed in the invention is suitable for allowing the growth layer to grow by 0.1 to 0.3 nm for a conventional wafer surface of a circular wafer with a diameter of 200 to 300 mm with 1 monolayer (ML) of water.

According to the invention, it is therefore provided in particular that at least 2 ML, preferably at least 5 ML, even more preferably at least 10 ML of fluid, especially water, be stored in the reservoir.

The formation of the reservoir by exposure to plasma is especially preferable, since plasma exposure moreover causes smoothing of the contact surface and hydrophilization as synergy effects. The surface is smoothed by plasma activation predominantly by a viscous flow of the material of the reservoir formation layer and optionally of the reaction layer. The increase of the hydrophilicity takes place especially by the increase of the silicon hydroxyl compounds, preferably by cracking of Si—O compounds present on the surface, such as Si—O—Si, especially according to the following reaction: Si—O—Si+H₂O

2SiOH

Another side effect, especially as a result of the aforementioned effects, comprises in that the prebond strength is improved especially by a factor of 2 to 3.

The reservoir in the reservoir formation layer on the first contact surface of the first substrate (and optionally of a reservoir formation layer on the second contact surface of the second substrate) is formed for example by plasma activation of the first substrate which has been coated with a thermal oxide. The plasma activation is carried out in a vacuum chamber in order to be able to adjust the conditions necessary for the plasma. According to the invention, N₂ gas, O₂ gas or argon gas with ion energies in the range from 0 to 2000 eV is used for the plasma discharge, as a result of which a reservoir is produced with a depth of up to 20 nm, preferably up to 15 nm, more preferably up to 10 nm, most preferably up to 5 nm, of the treated surface, in this case the first contact surface. In accordance with the invention, any particle type, atoms and/or molecules which are suitable for producing the reservoir can be used. Preferably those atoms and/or molecules are used which the reservoir produces with the required properties. The relevant properties are mainly the pore size, pore distribution and pore density. Alternatively, as claimed in the invention gas mixtures such as for example air or forming gas consisting of 95% Ar and 5% H₂ can be used. Depending on the gas used, in the reservoir during the plasma treatment, the following ions are present among others: N+, N₂+, O+, O₂+, Ar+. The first educt can be accommodated in the unoccupied free space of the reservoir/reservoirs. The reservoir formation layer and accordingly the reservoir can extend into the reaction layer.

Advantageously there are different types of plasma species which can react with the reaction layer and which comprise at least partially, preferably mostly of the first educt. To the extent the second educt is Si/Si, an O_(x) plasma species would be advantageous.

The reservoir/reservoirs is/are formed based on the following considerations: The pore size is smaller than 10 nm, preferably smaller than 5 nm, more preferably smaller than 1 nm, even more preferably smaller than 0.5 nm, most preferably smaller than 0.2 nm.

The pore density is preferably directly proportional to the density of the particles which produce the pores by striking action, most preferably can even be varied by the partial pressure of the striking species, and depending on the treatment time and the parameters, especially of the plasma system used.

The pore distribution preferably has at least one region of greatest pore concentration under the surface, by variation of the parameters of several such regions which are superimposed into a preferably plateau-shaped region (see FIG. 7). The pore distribution converges toward zero with increasing depth. The region near the surface during bombardment has a pore density which is almost identical to the pore density near the surface. After the end of plasma treatment the pore density on the surface can be reduced as a result of stress relaxation mechanisms. The pore distribution in the thickness direction with respect to the surface has one steep flank and with respect to the bulk a rather flatter, but continuously decreasing flank (see FIG. 7).

For the pore size, the pore distribution and pore density, similar considerations apply to all methods not produced with plasma.

The reservoir can be designed by controlled use and combination of process parameters. FIG. 7 shows a representation of the concentration of injected nitrogen atoms by plasma as a function of the penetration depth into a silicon oxide layer. It was possible to produce two profiles by variation of the physical parameters. The first profile 11 was produced by more highly accelerated atoms more deeply in the silicon oxide, conversely the profile 12 was produced after altering the process parameters in a lower density. The superposition of the two profiles yields a sum curve 13 which is characteristic for the reservoir. The relationship between the concentration of the injected atom and/or molecule species is evident. Higher concentrations designate regions with higher defect structure, therefore more space to accommodate the subsequent educt. A continuous change of the process parameters which is controlled especially in a dedicated manner during the plasma activation makes it possible to achieve a reservoir with a distribution of the added ions over the depth, which (distribution) is as uniform as possible.

As a reservoir, alternatively to a reservoir produced by plasma, the use of a TEOS (tetraethyl orthosilicate)-oxide layer on at least one of the substrates, at least the first substrate, is conceivable. This oxide is generally less dense than thermal oxide, for which reason compaction is advantageous as claimed in the invention. The compaction takes place by heat treatment with the objective of setting a defined porosity of the reservoir.

According to one embodiment of the invention, the filling of the reservoir can take place especially advantageously at the same time with the formation of the reservoir by the reservoir being applied as a coating to the first substrate, the coating already encompassing the first educt.

The reservoir is conceivable as a porous layer with a porosity in the nanometer range or as a layer which has channels with a channel density smaller than 10 nm, more preferably smaller than 5 nm, even more preferably smaller than 2 nm, most preferably smaller than 1 nm, most preferably of all smaller than 0.5 nm.

For the step of filling of the reservoir with a first educt or a first group of educts, as claimed in the invention the following embodiments, also in combination, are conceivable:

-   -   exposing the reservoir to the ambient atmosphere,     -   flushing with especially deionized water,     -   flushing with a fluid which contains the educt or which consists         of it, especially H₂O, H₂O₂, NH₄OH     -   exposing the reservoir to any gas atmosphere, especially atomic         gas, molecular gas, gas mixtures,     -   exposing the reservoir to a water vapor- or hydrogen peroxide         vapor-containing atmosphere and     -   depositing a reservoir already filled with the educt as a         reservoir formation layer on the first substrate.

The following compounds are possible as educts: O_(x) ⁺, O₂, O₃, N₂, NH₃, H₂O, H₂O₂, and/or NH₄OH.

The use of the above cited hydrogen peroxide vapor is regarded as the preferred version in addition to using water. Hydrogen peroxide furthermore has the advantage of having a greater oxygen to hydrogen ratio. Furthermore, hydrogen peroxide dissociates above certain temperatures and/or via the use of high frequency fields in the MHz range into hydrogen and oxygen.

Mainly when using materials with different coefficients of thermal expansion the use of methods for dissociation of the aforementioned species which do not cause any noteworthy temperature increase or at best a local/specific temperature increase is advantageous. In particular there is microwave irradiation which at least promotes, preferably causes the dissociation.

According to one advantageous embodiment of the invention, the formation of the growth layer and strengthening of the irreversible bond takes place by diffusion of the first educt into the reaction layer.

According to another advantageous embodiment of the invention, the formation of the irreversible bond takes place at a temperature of typically less than 300° C., advantageously less than 200° C., more preferably less than 150° C., even more preferably less than 100° C., most preferably at room temperature, especially during a maximum 12 days, more preferably a maximum 1 day, even more preferably a maximum 1 hour, most preferably a maximum 15 minutes. Another advantageous heat treatment method is dielectric heating by microwaves.

Here it is especially advantageous if the irreversible bond has a bond strength of greater than 1.5 J/m², especially greater than 2 J/m², preferably greater than 2.5 J/m².

The bond strength can be increased especially advantageously in that during the reaction, as claimed in the invention a product with a greater molar volume than the molar volume of the second educt is formed in the reaction layer. In this way growth on the second substrate is effected, as a result of which gaps between the contact surfaces can be closed by the chemical reaction as claimed in the invention. As a result, the distance between the contact surfaces, therefore the average distance, is reduced, and dead spaces are minimized.

To the extent the formation of the reservoir takes place by plasma activation, especially with an activation frequency between 10 and 600 kHz and/or a power density between 0.075 and 0.2 watt/cm² and/or with pressurization with a pressure between 0.1 and 0.6 mbar, additional effects such as smoothing of the contact surface and also a clearly increased hydrophilicity of the contact surface are effected.

Alternatively thereto, the formation of the reservoir can take place as claimed in the invention by use of a tetraethoxysilane oxide layer compacted in a controlled manner to a certain porosity as reservoir formation layer.

According to another advantageous embodiment of the invention it is provided that the reservoir formation layer is comprised largely, especially essentially completely, of an especially amorphous silicon dioxide, especially one produced by thermal oxidation, and the reaction layer consists of an oxidizable material, especially predominantly, preferably essentially completely, of Si, Ge, InP, GaP or GaN (or another material mentioned alternatively above). An especially stable reaction which especially effectively closes the existing gaps is enabled by oxidation.

Here it is provided as claimed in the invention that there is a growth layer, especially predominantly of native silicon dioxide (or another material mentioned alternatively above) between the second contact surface and the reaction layer. The growth layer is subject to growth caused by the reaction as claimed in the invention. The growth takes place proceeding from the transition Si—SiO2 (7) by re-formation of amorphous SiO2 and the deformation caused thereby, especially bulging, of the growth layer, especially on the interface to the reaction layer, and especially in regions of gaps between the first and the second contact surface. This causes a reduction of the distance or a reduction of the dead space between the two contact surfaces, as a result of which the bond strength between the two substrates is increased. A temperature between 200 and 400° C., preferably roughly 200° C. and 150° C., more preferably a temperature between 150° C. and 100° C., most preferably a temperature between 100° C. and room temperature, is especially advantageous. The growth layer can be divided into several growth regions. The growth layer can at the same time be a reservoir formation layer of the second substrate in which another reservoir which accelerates the reaction is formed.

Here it is especially advantageous if the growth layer has an average thickness “A” between 0.1 nm and 5 nm prior to formation of the irreversible bond. The thinner the growth layer, the more quickly and easily the reaction takes place between the first and the second educt through the growth layer, especially by diffusion of the first educt through the growth layer to the reaction layer. The diffusion rate of the educts through the growth layer is increased by the growth layer which has been thinned and thus is very thin at least at the beginning of the formation of the permanent bond or at the start of the reaction. This leads to a lower transport time of the educts at the same temperature.

Here the thinning as claimed in the invention can play a decisive part since the reaction can be further accelerated hereby and/or the temperature can be further reduced. Thinning can be done especially by etching, preferably in a moist atmosphere, still more preferably in-situ. Alternatively thinning takes place especially by dry etching, preferably in-situ. Here in-situ means performance in the same chamber in which at least one previous and/or one following step is/are done. Another system arrangement which is included in the concept of in-situ used here is a system in which the transport of the substrates between individual process chambers can take place in an atmosphere which can be adjusted in a controlled manner, for example using inert gases, but especially in a vacuum environment. Wet etching takes place with chemicals in the vapor phase, while dry etching takes place with chemicals in the liquid state. To the extent the growth layer consists of silicon dioxide, etching with hydrofluoric acid or diluted hydrofluoric acid can be done. To the extent the growth layer consists of pure Si, etching can be done with KOH.

According to one embodiment of the invention it is advantageously provided that the formation of the reservoir is carried out in a vacuum. Thus contamination of the reservoir with unwanted materials or compounds can be avoided.

In another embodiment of the invention, it is advantageously provided that filling of the reservoir takes place by one or more of the steps cited below:

-   -   exposing the first contact surface to the atmosphere, for         filling the reservoir with atmospheric humidity and/or oxygen         contained in the air,     -   exposing the first contact surface to a fluid comprised         especially predominantly, preferably almost completely, of         especially deionized H₂O and/or H₂O₂,     -   exposing the first contact surface to N₂ gas and/or O₂ gas         and/or Ar gas and/or forming gas, especially consisting of 95%         Ar and 5% H₂, especially with an ion energy in the range from 0         to 2000 eV,     -   vapor deposition for filling the reservoir with any already         named educt.

It is especially effective for the process sequence if the reservoir is formed preferably in a thickness R between 0.1 nm and 25 nm, more preferably between 0.1 nm and 15 nm, even more preferably between 0.1 nm and 10 nm, most preferably between 0.1 nm and 5 nm. Furthermore, according to one embodiment of the invention it is advantageous if the average distance B between the reservoir and the reaction layer immediately before formation of the irreversible bond is between 0.1 nm and 15 nm, especially between 0.5 nm and 5 nm, preferably between 0.5 nm and 3 nm. The distance B as claimed in the invention is influenced or produced by thinning.

A device for executing the method is formed as claimed in the invention with a chamber for forming the reservoir, a chamber provided especially separately for filling the reservoir, and an especially separately provided chamber for forming the prebond, all of which chambers are connected directly to one another via a vacuum system.

In another embodiment the filling of the reservoir can also take place directly via the atmosphere, therefore either in a chamber which can be opened to the atmosphere or simply on a structure which does not have jacketing but can handle the wafer semiautomatically and/or completely automatically.

Other advantages, features and details of the invention will become apparent from the following description of preferred exemplary embodiments and using the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a first step of the method as claimed in the invention immediately after the first substrate makes contact with the second substrate,

FIG. 1 b shows an alternative first step of the method as claimed in the invention immediately after the first substrate makes contact with the second substrate,

FIG. 2 shows a step of the process as claimed in the invention which takes place before contact-making, specifically thinning of the second substrate,

FIGS. 3 a and 3 b show other steps of the method as claimed in the invention for forming a higher bond strength,

FIG. 4 shows another step of the method as claimed in the invention which follows the steps according to FIG. 1 a or 1 b, FIG. 2, and FIGS. 3 a and 3 b, with substrate contact surfaces which are in contact,

FIG. 5 shows a step as claimed in the invention for formation of an irreversible/permanent bond between the substrates,

FIG. 6 shows an enlargement of the chemical/physical processes which proceed on the two contact surfaces during the steps according to FIG. 4 and FIG. 5,

FIG. 7 shows a further enlargement of the chemical/physical processes which proceed on the interface between the two contact surfaces during the steps according to FIG. 4 and FIG. 5 and

FIG. 8 shows a diagram of the production of the reservoir as claimed in the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The same or equivalent features are identified with the same reference numbers in the figures.

In the situation shown in FIG. 1, only one extract of the chemical reactions which proceed during or immediately after the prebond step between a first contact surface 3 of a first substrate 1 and a second contact surface 4 of a second substrate 2 is shown [sic]. The surfaces are terminated with polar OH groups and are accordingly hydrophilic. The first substrate 1 and the second substrate 2 are held by the force of attraction of the hydrogen bridges between the OH groups present on the surface and the H₂O molecules and also between the H₂O molecules alone. The hydrophilicity of at least the first substrate 1 has been increased by plasma treatment in a preceding step.

It is especially advantageous to additionally subject the second substrate 2 or the second contact surface 4 to plasma treatment, especially at the same time with the plasma treatment of the first substrate 1 according to the alternative embodiment.

A reservoir 5 in a reservoir formation layer 6 comprised of thermal silicon dioxide as well as in the alternative embodiment according to FIG. 1 b a second opposing reservoir 5′ in the reservoir formation layer 6′ have been formed as claimed in the invention by plasma treatment. Under the reservoir formation layers 6, 6′ reaction layers 7, 7′ which contain a second educt or a second group of educts directly adjoin. Plasma treatment with O₂ ions with ion energies in the range between 0 and 2000 eV yields an average thickness R of the reservoir 5 of roughly 15 nm, the ions forming channels or pores in the reservoir formation layer 6.

Between the reservoir formation layer 6 and the reaction layer 7 there is a growth layer 8 on the second substrate 2 which can be at the same time at least partially the reservoir formation layer 6′. Accordingly there can additionally be another growth layer between the reservoir formation layer 6′ and the reaction layer 7′.

Likewise the reservoir 5 (and optionally the reservoir 5′) is filled at least largely with H₂O as the first educt prior to the step shown in FIG. 1 and after plasma treatment. Reduced species of the ions present in the plasma process can also be located in the reservoir, especially O₂, N₂, H₂, Ar.

Before or after the formation of the reservoir/reservoirs 5, 5′, in any case before the substrates 1, 2 make contact, the growth layer 8 (and optionally the other growth layer) is thinned by etching (here after the formation of the reservoir 5, see FIG. 2). In this way the average distance B between the second contact surface 4 and the reaction layer 7 is reduced. At the same time the second contact surface 4 is advantageously flatter.

The contact surfaces 3, 4 still have a relatively wide gap, especially dictated by the water present between the contact surfaces 3, 4, after making contact in the stage shown in FIGS. 1 a and 1 b. Accordingly the existing bond strength is relatively low and is roughly between 100 mJ/cm² and 300 mJ/cm², especially more than 200 mJ/cm². In this connection the prior plasma activation plays a decisive part, especially due to the increased hydrophilicity of the plasma-activated first contact surface 3 and a smoothing effect which is caused by the plasma activation.

The process which is shown in FIG. 1 and which is called prebond can preferably proceed at ambient temperature or a maximum 50° Celsius. FIGS. 3 a and 3 b show a hydrophilic bond, the Si—O—Si bridge arising with splitting of water by —OH terminated surfaces. The processes in FIGS. 3 a and 3 b last roughly 300 h at room temperature. At 50° C. roughly 60 h. The state in FIG. 3 b occurs at the indicated temperatures without producing the reservoir 5 (or reservoirs 5, 5′).

Between the contact surfaces 3, 4, H₂O molecules are formed and provide at least partially for further filling in the reservoir 5 to the extent there is still free space. The other H₂O molecules are removed. In the step according to FIG. 1 roughly 3 to 5 individual layers of OH groups or H₂O are present and 1 to 3 monolayers of H₂O are removed or accommodated in the reservoir 5 from the step according to FIG. 1 to the step according to FIG. 3 a.

In the step shown in FIG. 3 a, the hydrogen bridge bonds are now formed directly between siloxane groups, as a result of which a greater bond force arises. This draws the contact surfaces 3, 4 more strongly to one another and reduces the distance between the contact surfaces 3, 4. Accordingly there are only 1 to 2 individual layers of OH groups between the contact surfaces 1, 2.

In the step shown in FIG. 3 b, in turn with separation of H₂O molecules according to the reaction which has been inserted below, covalent compounds in the form of silanol groups are now formed between the contact surfaces 3, 4 which lead to a much stronger bond force and require less space so that the distance between the contact surfaces 3, 4 is further reduced until finally the minimum distance shown in FIG. 3 is reached based on the contact surfaces 3, 4 directly meeting one another: Si—OH+HO—Si

Si—O—Si+H₂O

Up to stage 3, especially due to the formation of the reservoir 5 (and optionally of the additional reservoir 5′), it is not necessary to unduly increase the temperature, rather to allow it to proceed even at room temperature. In this way an especially careful progression of the process steps according to FIG. 1 a or 1 b to FIG. 4 is possible.

In the method step shown in FIG. 5, the temperature is preferably increased to a maximum 500° Celsius, more preferably to a maximum 300° C., even more preferably to a maximum 200° C., most preferably to a maximum 100° C., most preferably of all not above room temperature in order to form an irreversible or permanent bond between the first and the second contact surface. These temperatures which are relatively low, in contrast to the prior art, are only possible because the reservoir 5 (and optionally in addition the reservoir 5′) encompasses the first educt for the reaction shown in FIGS. 6 and 7: Si+2H₂O→SiO₂+2H₂

By increasing the molar volume and diffusion of the H₂O molecules, especially on the interface between the reservoir formation layer 6′ and the reaction layer 7 (and optionally in addition on the interface between the reservoir formation layer 6 and the reaction layer 7′), volume in the form of a growth layer 8 increases, due to the objective of minimizing the free Gibbs enthalpy enhanced growth taking place in regions where gaps 9 are present between the contact surfaces 3, 4. The gaps 9 are closed by the increase in the volume of the growth layer 8. More exactly:

At the aforementioned slightly increased temperatures, H₂O molecules diffuse as the first educt from the reservoir 5 (or the reservoirs 5, 5′) to the reaction layer 7 (and optionally 7′). This diffusion can take place either via a direct contact of the reservoir formation layer 6, 6′ which is formed as oxide layers with the respective reaction layer 7, 7′ (or growth layer 8) or via a gap 9 or from a gap 9 which is present between the oxide layers. There, silicon oxide, therefore a chemical compound with a greater molar volume than pure silicon, is formed as a reaction product 10 of the aforementioned reaction from the reaction layer 7. The silicon dioxide grows on the interface of the reaction layer 7 with the growth layer 8 and/or the reservoir formation layer 6, 6′ and thus forms the growth layer 8 which is made especially as native oxide in the direction of the gaps 9. Here H₂O molecules from the reservoir are also required.

Due to the existence of the gaps which are in the nanometer range, there is the possibility of bulging of the growth layer 8, as a result of which stresses on the contact surfaces 3, 4 can be reduced. In this way the distance between the contact surfaces 3, 4 is reduced, as a result of which the active contact surface and thus the bond strength are further increased. The weld connection which has arisen in this way, which closes all pores, and which forms over the entire wafer, in contrast to the products in the prior art which are partially not welded, fundamentally contributes to increasing the bond force. The type of bond between the two amorphous silicon oxide surfaces which are welded to one another is a mixed form of covalent and ionic portion.

The aforementioned reaction of the first educt (H₂O) with the second educt (Si) takes place in the reaction layer 7 especially quickly or at temperatures as low as possible to the extent an average distance B between the first contact surface 3 and the reaction layer 7 is as small as possible.

Therefore the pretreatment of the first substrate 1 and the choice/pretreatment of the second substrate 2 which is comprised of a reaction layer 7 of silicon and a native oxide layer as thin as possible as growth layer 8 are decisive. There is a native oxide layer as thin as possible for two reasons. The growth layer 8 is very thin, especially due to the thinning provided as claimed in the invention, so that due to the newly formed reaction product 10 on the reaction layer 7 it can bulge toward the reservoir formation layer 6 of the opposite substrate 1, which layer is made as an oxide layer, predominantly in regions of the nanogaps 9. Furthermore, diffusion paths as short as possible are desired in order to achieve the desired effect as quickly as possible and at a temperature as low as possible. The first substrate 1 likewise consists of a silicon layer and an oxide layer produced thereon as reservoir formation layer 6 in which a reservoir 5 is formed at least partially or completely.

The reservoir 5 (or the reservoirs 5, 5′) is filled as claimed in the invention at least with the amount of the first educt which is necessary to close the nanogaps 9 so that an optimum growth of the growth layer 8 can take place to close the nanogaps 9 in a time as short as possible and/or at a temperature as low as possible.

REFERENCE NUMBER LIST

-   1 first substrate -   2 second substrate -   3 first contact surface -   4 second contact surface -   5, 5′ reservoir -   6, 6′ reservoir formation layer -   7, 7′ reaction layer -   8 growth layer -   9 nanogaps -   10 reaction product -   11 first profile -   12 second profile -   13 sum curve -   A average thickness -   B average distance -   R average thickness 

The invention claimed is:
 1. A method for bonding a first contact surface of a first substrate to a second contact surface of a second substrate, the second substrate having at least one reaction layer containing a second educt or a second group of educts, said method comprised of the following steps: (a) forming a reservoir for retaining a first educt or a first group of educts in a reservoir formation layer on the first contact surface, (b) at least partially filling the reservoir with the first educt or the first group of educts after the reservoir is formed, (c) thinning the second substrate, (d) after the reservoir is filled with the first educt or the first group of educts, forming a prebond connection by bringing the first contact surface and the second contact surface into contact with each other, and (e) forming a permanent bond between the first contact surface and the second contact surface, said permanent bond at least partially strengthened by a reaction of the first educt or the first group of educts with the second educt or the second group of educts contained in the at least one reaction layer of the second substrate, wherein an average distance between the reservoir and the at least one reaction layer immediately before formation of the permanent bond is between 0.1 nm and 15 nm.
 2. The method as claimed in claim 1, wherein thinning the second substrate includes etching.
 3. The method as claimed in claim 1, wherein thinning the second substrate includes dry etching.
 4. The method as claimed in claim 1, wherein said method further comprises forming a reservoir in a reservoir formation layer on the second contact surface of the second substrate.
 5. The method as claimed in claim 1, wherein during the reaction of the first educt or the first group of educts with the second educt or the second group of educts, a reaction product having a molar volume that is greater than a molar volume of the second educt or the second group of educts is formed in the at least one reaction layer.
 6. The method as claimed in claim 1, wherein the reservoir is formed by plasma activation.
 7. The method as claimed in claim 1, wherein the reservoir is formed by using compacted tetraethoxysilane oxide layer as the reservoir formation layer.
 8. The method as claimed in claim 1, wherein the reservoir formation layer is comprised of an amorphous material, and the at least one reaction layer is comprised of an oxidizable material.
 9. The method as claimed in claim 1, wherein a growth layer is disposed between the second contact surface and the at least one reaction layer, and the growth layer is at least partially removed by the thinning of the second substrate.
 10. The method as claimed in claim 9, wherein before the forming of the permanent bond and after the thinning of the second substrate, the growth layer has an average thickness “A” between 1 angstrom and 10 nm.
 11. The method as claimed in claim 1, wherein the forming of the reservoir includes forming the reservoir in a vacuum.
 12. The method as claimed in claim 1, wherein the reservoir is filled by one or more of the following steps: exposing the first contact surface to the atmosphere, exposing the first contact surface to a fluid comprising deionized H₂O and/or H₂O₂, exposing the first contact surface to N₂ gas and/or O₂ gas and/or Ar gas and/or a forming gas with an ion energy in the range from 0 to 200 eV.
 13. The method as claimed in claim 1, wherein the reservoir has an average thickness between 0.1 nm and 25 nm.
 14. The method as claimed in claim 1, wherein an average distance between the reservoir and the at least one reaction layer is between 0.5 nm and 5 nm.
 15. The method as claimed in claim 1, wherein the permanent bond has a bond strength that is twice a bond strength of the prebond.
 16. The method as claimed in claim 2, wherein thinning the second substrate includes etching in a moist atmosphere.
 17. The method as claimed in claim 4, wherein the reservoir in the reservoir formation layer on the second contact surface is formed by plasma activation.
 18. The method as claimed in claim 4, wherein the reservoir in the reservoir formation layer on the second contact surface is formed by using compacted tetraethoxysilane oxide layer as the reservoir formation layer.
 19. The method as claimed in claim 14, wherein the average distance is between 0.5 nm and 3 nm. 